We report on investigations of type I clathrate Si and Ge compounds with Ba partially substituted by rare earth atoms. Novel compounds from framework-deficient solid solutions Ba8 Alx Si42−3/4x □4−1/4x and Ba8 Gax Si42−3/4x □4−1/4x (x = 8, 12, 16; □, open square... lattice defect) have been prepared and characterized. All x-ray intensity data are consistent with the standardized clathrate I-Ba8Al16Ge30 type structure (space group Pm3̄n). In rare earth substituted clathrates, Eu2Ba6MxSi46−x (M = Cu, Al, Ga), rare earth atoms completely occupy the 2a position and thus form a new quaternary ordered version of the Ba8Al16Ge30 structure type. From a geometrical analysis of clathrate crystal structures, a systematic scheme for all known clathrate compounds is proposed. All clathrates studied are metals with low electrical conductivity. The highest Seebeck coefficient in the present series is deduced for Ba8In16Ge30, S = −75μV K−1, indicating transport processes dominated by electrons as carriers. The Eu-based clathrates investigated exhibit long-range magnetic order as high as 32 K for Eu2Ba6Al8Si36 of presumably ferromagnetic type. Magnetic susceptibility indicates in all cases a 2+ ground state for the Eu ions, in fine agreement with LIII absorption edge spectra.

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Thermal conductivity of superconducting MgB 2 was studied in both the superconducting and the normal state region. The latter is almost equally determined by the electronic -and the lattice contribution to the total thermal conductivity. In the superconducting state, however, the lattice contribution is larger. The electronic thermal conductivity below T c was derived from the experimental data considering the Bardeen-Rickayzen-Tewordt theory together with the model of Geilikman. The analysis shows that electron scattering on static imperfections dominates.

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